1. Field of the Invention
This invention relates to piezoelectric materials and more particularly, to piezoelectric polymer materials which are useful in detection of a grasping force of a manipulator such as of robots and are also useful as a tweeter or an ultrasonic probe for medical purposes. The invention also relates to a method for making such piezoelectric polymer materials.
2. Description of the Prior Art
In recent years, piezoelectric pressure-sensitive elements have wide utility in various fields. These elements make use of a piezoelectric effect in which when a pressure is applied to a material, dielectric polarization takes place along a certain direction, i.e. a positive charge appears at one end with a negative charge developing at the other end. When this piezoelectric effect is utilized, mechanical signals expressed by pressure can be converted into electric signals.
Although piezoelectric polymer materials do not necessarily have very high piezoelectric characteristics, they have a number of advantages in practical applications over known piezoelectric ceramic materials because of their flexibility and capability of forming a large-size or thin film or sheet.
Known piezoelectric polymer materials are, for example, polyvinylidene fluoride (PVDF), copolymers of vinylidene fluoride and trifluoroethylene (VDF/TrFE), and copolymers of vinylidene fluoride and tetrafluoroethylene (VDF/TeFE), and copolymers of vinylidene cyanide and vinyl acetate (VDCN/VA).
However, these known piezoelectric polymer materials have heat resistances as low as approximately 110.degree. C. In addition, they are thermoplastic resins with a linear molecular structure, so that their adhesion to other materials is very poor. Especially, the poor adhesion to other materials presents a serious obstacle in the formation of electrodes when pressures are detected as electric signals. More particularly, the formation of electrodes by adhesion of a conductive, pane or a metallic foil to piezoelectric polymer materials such as PVDF is substantially impossible. The formation of electrodes is usually effected by deposition of, for example, Al. However, the adhesion strength of the electrodes formed by vacuum deposition is not so great. Accordingly, when electrodes are formed on a piezoelectric polymer material and a stress such as pressure is applied thereto over a long time, the electrodes are liable to separate from the piezoelectic material, making it impossible to detect the stress.
On the other hand, polymers, which exhibit good adhesion to other materials to allow relatively easy formation of electrodes, include thermosetting resins such as epoxy resins, phenolic resins and the like. However, these resins are substantially amorphous in nature, so that even though a dipolar orientation operation is effected by application of a high electric field, piezoelectricity is not shown. Accordingly, development of a piezoelectric polymer material from thermosetting polymers has never been accomplished.